Navigation systems using radio signals have been around for many years, and today the GPS system (Global Satellite Positioning) is arguably the best known, being the first satellite system to be available and widely used. Such satellite navigation systems are widely referred to within the general class of GNSS's (Global Satellite Navigation Systems) of which Glonass and Galileo are two further examples.
In addition to satellite navigation systems, there are a number of examples of radio location systems based on terrestrial radio signals. These include the E-OTD (Enhanced Observed Time Difference of Arrival) system used on GSM, UWB (Ultrawideband) systems, Wi-fi and ZigBee systems and many others within the general category of RFID (radio frequency identification), generally referred to as RTLS's (Real-Time Location Systems).
GNSS's provide global coverage for receivers that are able to receive the satellite signals (sufficient “visibility” of the sky) and inherent accuracies of about ten (10) metres. The basic accuracy of these systems is affected by three main classes of errors:                1. Satellite errors, including clock errors and imprecision in knowing the positions of the satellites when the navigation signals were transmitted;        2. Radio propagation errors in both ionosphere and troposphere; and        3. Receiver errors.        
Many different techniques have been developed, and continue to be developed, in order to characterise and minimise the impact of these errors and thereby improve receiver performance in terms of accuracy, or alternatively better coverage into areas where the received signals are very weak and may otherwise be unusable.
One of the core methods used to improve performance is to make relative measurements and to use the principle of differential measurements. This approach is sometimes referred to as “double differencing”. The concept is to place a first receiver at a known position; make measurements of the received signals; compute the satellite and radio link errors in these measurements and send the errors to a second receiver which can correct its own measurements using the correction terms and thereby achieve a position determination which suffers much less from satellite and radio link errors. Double differencing techniques like these are so effective that they can allow very precise measurements to be made using the signal carrier phase in addition to the basic code phase measurements.
U.S. Pat. No. 4,751,512 “Differential navigation system for remote mobile users” describes the process of differential positioning. The techniques are also widely described and used for GPS systems, such as in U.S. Pat. No. 5,148,179 “Differential position determination using satellites” and U.S. Pat. No. 5,442,363 “Kinematic global positioning system of an on-the-fly apparatus for centimeter-level positioning for static or moving applications”. It has also been applied to particular GPS applications such as, for example, in U.S. Pat. No. 5,361,212 “Differential GPS landing assistance system”, and U.S. Pat. No. 5,469,175 “System and method for measuring distance between two objects on a golf course”.
Differential GPS systems are now widely deployed for commercial use sometimes using a locally installed reference receiver and sometimes using commercial or publicly broadcast differential correction data on services such as WAAS (Wide Area Augmentation System) and EGNOS (European Geostationary Navigation Overlay System).
Whereas all of the systems previously mentioned require the use of a reference receiver at a known location, other systems that do not have such a reference receiver are described for both one and two receiver configurations, as for example in U.S. Pat. No. 6,397,147 “Relative GPS positioning using a single GPS receiver with internally generated differential correction terms”. This approach gives excellent relative accuracy, but both positions still have the same uncertainty as a standard GPS position fix.
Another popular way of improving the performance of GNSS's is to use different complementary technology, such as inertial navigation techniques or other radio location systems as an aid, particularly in areas with poor satellite signal coverage.
Assisted GPS is now widely used for commercial handheld mobile communications devices as a way of helping the GPS receiver to obtain a position fix under conditions where it would otherwise likely fail, or for helping it to acquire the satellite signals more quickly. Examples of these techniques are described in U.S. Pat. No. 7,295,156 “Cell phone GPS positioning system”, U.S. Pat. No. 7,283,091 “Radio positioning system for providing position and time for assisting GPS signal acquisition in a mobile unit” and others.
Inertial systems, accelerometers and rate gyroscopes, are widely used to assist GPS. Initially such systems were only used in high performance military systems, but now low cost inertial systems are becoming common. Examples of such systems are described in U.S. Pat. No. 6,721,657 “Inertial GPS navigation system” and U.S. Pat. No. 5,657,025 “Integrated GPS/inertial navigation apparatus providing improved heading estimates”.
There is, however, a continuing need for improved techniques.